Little Mac huh? Well, let's see...
Zelda being more of a grounded character, she doesn't have very good odds going toe to toe against Little Mac. He's faster than her in every way, and he's got super armor all over the place. You definitely can't afford to throw out any "useless" movements here, as he'll most likely punish you very promptly for doing so.
There is still hope though! You have to abuse of the fact that he has an absolutely horrible, horrible recovery. So early gimping will be your best friend. One sweetspotted Dair offstage is pretty much the end of Little Mac at any % really:
https://www.youtube.com/watch?v=BWrbBw--YnA <= As evidenced here.
And why wouldn't you pursue him off stage? There is very little risk involved as his aerials are all abysmal, and really the only notable dangers would be:
1. Counter, but that's very risky for him, considering he still needs to claw his way back to the stage and will probably only attempt this if he's recovering from high up.
2. Side B, which really he will only do when he's close to the stage because it puts him in helpless after words. In fact, you can probably bait this and punish him back on stage since it has a small travel distance and has very noticeable landing lag. Unless of course he's trying to sweetspot the ledge, but that sets him up for an open Dair gimp if he's not careful.
So definitely when you get him off stage, DO NOT BE AFRAID TO TRY AND GIMP HIM! Zelda's recovery has amazing distance, so you can go in pretty deep for the gimp if you have to. The earlier you end his stock the better as it also resets his KO Punch meter.
Speaking of his KO Punch, if it does get full don't panic. All you need to do is smack him for 10% or more damage and he loses it! Just be aware to not use anything that'll leave you totally open and try to only take action when he makes himself vulnerable. Things like missed Nayru's or Smashes are really going to be your grave in this situation and you're both going to probably be playing very carefully once the KO Punch is charged up, so you're gonna have to be patient.
Or if you're already high %, Little Mac might try to end your stock ASAP so he can use a clean KO Punch on your fresh stock. Note that from 0-15%ish (possibly more with Vectoring), KO Punch will not kill you, so if you're on a fresh stock and Mac's got KO Punch, use this opportunity to knock it out of him! And if he hits you with it early, at said 0-15%~ it's WAY better to take the 35% damage than a stock, not to mention he's gonna have to build it up from scratch again. So keep these things in mind.
Also KO Punch in the air is utterly, utterly useless. It has no range, does only 13% and absolutely no knockback. Mac in the air = Yes for you.
Probably the biggest annoyances are gonna be his incredible ground normals (naturally).
- Jab is gonna probably be his main damage builder. Speedy and painful. If you end up shielding up to the point of his rapid jab part, try to hold out until you see the final punch part; you should be able to sneak in a dash attack or grab.
- Ftilt inches him forwards and definitely has a semi decent amount of kill power and is fast, so if you somehow manage to live up to a higher % be on the look out for this one.
- Fsmash whiffs if Little Mac is right next to you, unless he uses the Uppercut version. The power and super armor goes without saying.
- Usmash does not hit behind him very well, if at all. HOWEVER it has a very painful sweetspot if you're right in front of him (it'll have fiery effect).
- Dsmash is ultra fast, but not quite as much KO power as his other two smashes. Just don't get caught by this near the ledge.
- Dash Attack won't kill you 'till later %, and has a surprisingly low lag period so retaliation needs to be quick. If you're by the ledge, Dash Attack will almost always put Little Mac BEHIND you so accommodate for that.
Speaking of the ledge, this is one of the better places for Zelda to be actually in a neutral setting. Because Little Mac has no projectiles, he is forced to approach Zelda with a lot of risks involved:
1. Zelda's Back throw will most definitely end a stock if he's remotely high %, if only because he won't be able to recover. And if not, it's a free Dair chance / Bait situation.
2. If he grabs you and throws you off stage, there is no risk of returning back as he can't gimp (unless you count that Nair lock, but that's gonna cost him a stock too) and he can't kill you really with a throw unless you're 180%+ with his Bthrow at the ledge or something. So really, you're still alive! Just don't EVER try to Farore's Wind onto the stage. Always go for the ledge.
3. Probably the situation you need to be most worried about is if he starts to walk up to you. Ftilt and the like with his speed might kill you early... so be prepared to hold shield a bunch and react accordingly!
Hmm, that's mostly what comes to mind. I hope that helps? I'm sure I missed a few things, so please add onto this if needed!